The invention relates to powder coatings in general and, more specifically, to powder coatings that provide a low gloss appearance to the coated article.
Powder coatings are widely used to provide a decorative and/or protective coating on a substrates. They are becoming increasingly popular because they are applied in a solid state or slurry. These application states mean that the powder coatings use little or no solvents, unlike their conventional liquid coating counterparts. In addition, solid state application permits the powder to be collected, purified and re-used.
In certain applications, it is necessary or desirable for the powder coating to have a surface that is smooth in appearance, but has a low gloss or shine. Such applications are those where low gloss is aesthetically desired, or where glare from the coating surface can interfere with the safe or proper use of the coated article, such as firearms, optical devices, military applications and motor vehicles, aircraft and other vehicles. Prior art attempts to control gloss in powder coatings has taken three different approaches using fillers, waxes and differential cure.
The addition of fillers is known to reduce the gloss of powder coatings. Indeed, gloss reduction is an unavoidable, and often undesirable, side effect of filler addition. For example, the 3M Company markets ceramic microspheres under the trade name Zeeospheres(trademark) for use in powder coatings to control gloss. A filler commonly used for gloss control is wollastonite, whose needle-shaped crystals are very effective at reducing gloss by reducing the microscopic smoothness of coatings. Fillers of other shapes are also commonly used to reduce gloss. The shortcoming of the use of fillers to control gloss is that their addition also reduces coating flow, typically increasing the amount of waviness or texture known as xe2x80x9corange peel.xe2x80x9d
Hydrocarbon and fluorocarbon waxes are used to reduce the gloss of powder coatings. As a wax-containing coating is baked, the wax migrates to the coating/air interface where it forms a layer with reduced gloss. Shortcomings of this approach are that the wax softens the coating surface and reduces its resistance to marring, staining and chemical attack.
Another way to reduce gloss, which is especially effective with epoxy and epoxy/polyester hybrid coatings, is to incorporate at least two curing agents or two differently structured or differently-catalyzed resins. Upon incomplete molecular mixing, such as is typically encountered in a powder coating extruder, these differential-cure systems result in the development of zones of varying shrinkage or varying surface tension on the coating surface during cure, yielding a microscopically-rough layer which is seen as low gloss.
Variations of this approach are widely used. A shortcoming of this approach is that coating properties such as impact resistance, flexibility, or chemical resistance suffer.
In one aspect, the invention provides an improved powder coating composition, the improvement wherein comprising the use in the composition of spheroidal particles having a mean particle size greater than 10 microns and preferably greater than 15 microns, and having a maximum particle size of about 50 microns.
In another embodiment, the invention provides a process of reducing gloss in a powder coating, the process comprising adding spheroidal particles to a powder coating composition, wherein said spheroidal particles have a mean particle size greater than 10 microns and preferably greater than 15 microns, and have a maximum particle size of about 50 microns.
These and other features of the invention will become apparent on a further reading of the application.
The powder coatings of this invention provide the formulator with an opportunity to control the gloss of the final coating while minimizing or eliminating the negative effects of the prior art attempts at controlling gloss; i.e., loss of coating flow and creation of xe2x80x9corange peelxe2x80x9d surface effects. It is important to note that the coatings of this invention have a rough or textured surface microscopically, but otherwise appear smooth to the naked eye.
The powder coating compositions of this invention contain one or more thermosetting or thermoplastic resins commonly used in such coatings and well known in the art. Such resins include those based on epoxy, polyester, acrylic and/or urethane resins. Examples of such resins include saturated and unsaturated polyesters, acrylics, acrylates, polyester-urethanes, acrylic-urethanes, epoxy, epoxy-polyester, polyester-acrylics and epoxy-acrylics. Useful thermoplastic resins may include nylon, polyvinylchloride, polyethylene, polyethylene terephthalate, polybutylene terphthalate and polypropylene, for example.
The powder coating compositions of this invention may be applied by electrostatic spray, thermal or flame spraying, or fluidized bed coating methods, all of which are known to those skilled in the art. The coatings may be applied to metallic and/or non-metallic substrates. Following deposition of the powder coating to the desired thickness, the coated substrate is typically heated to melt the composition and cause it to flow. In certain applications, the part to be coated may be pre-heated before the application of the powder, and then either heated after the application of the powder or not. Gas or electrical furnaces are commonly used for various heating steps, but other methods (e.g., microwave) are also known. Curing (i.e., cross-linking) of the coating may be a carried out by thermal or photochemical methods (e.g., ultraviolet radiation, infrared radiation, etc.). Curing may be effected by heat conduction, convection, radiation or any combination thereof.
The powder coating compositions of this invention contain spheroidal particles. The term xe2x80x9cspheroidalxe2x80x9d as used herein means generally spherical in shape. More specifically, the term means filler materials that contain less than 25% particle agglomerates or fractured particles containing sharp or rough edges. The spheroidal particles should be non-reactive or inert so as not to interfere with the other properties of the composition. Examples of suitable spheroidal particles are glass microspheres, ceramic microspheres, naturally-occurring or synthetic spheroidal minerals such as cristobalite, polymer microspheres and metal microspheres.
As already mentioned, the spheroid particles must have a mean particle size greater than 10 microns, preferably of greater than 15 microns. Intermediate ranges are included. As the mean particle diameter decreases, the surface per unit weight increases. The increase in surface area results in a tendency of the filler to dry the coating, reduce flow, and induce roughness in the coating. As indicated in the working examples, spheroidal particles having a mean diameter of 10 microns or below produced only marginal results in gloss control, whereas at mean diameters greater than 10, particularly of greater than 15, the spheroidal particles gave good results.
The upper limit of the diameter of the spheroidal particles is dependent on the intended thickness of the final coating in that the particles must have a diameter less than the coating thickness. Most powder coatings, especially xe2x80x9cdecorativexe2x80x9d powder coatings, are designed to be applied at a dry film thickness of about 50 microns. Thus, in most applications, the spheroidal particles should have a maximum diameter of less than about 50 microns, preferably 40 microns.
The spheroidal particles may be present in the composition in an amount of from 5 wt % to 60 wt %, based on the total weight of the powder coating composition. Below 5 wt %, little effect on gloss is observed. Above 60 wt %, an unacceptable loss of coating flow results. It is understood that these are general guidelines and the exact weight % of spheroidal particles will depend on the specific gravity of the spheroidal particles, the degree of gloss reduction desired and the other components of the powder coating composition.
In addition to the resins and spheroidal particles, the powder coating compositions of this invention may contain other additives that are conventionally used in powder coating compositions. Examples of such additives include fillers, extenders, flow additives, catalysts, hardeners and pigments. Compounds having anti-microbial activity may also be added as is taught in U.S. Pat. No. 6,093,407, the entire disclosure of which is incorporated herein by reference.
The powder coatings of this invention are prepared by conventional manufacturing techniques used in the powder coating industry. For example, the ingredients used in the powder coating, including the spheroidal particles, can be blended together and heated to a temperature to melt the mixture and then extruded. The extruded material is then cooled on chill rolls, broken up and then ground to a fine powder.
The spheroidal particles may also be combined with the coating powder after it is formed in a process known as xe2x80x9cbonding.xe2x80x9d In this process, the coating powder and the material to be xe2x80x9cbondedxe2x80x9d with it are blended and subjected to heating and impact fusion to join the differing particles.